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Sleep's role in memory consolidation unpicked

Sleep spindles, a type of brain activity recorded during stage two of sleep, has repeatedly been associated with improving brain plasticity and consolidating memory. Scientists are now drawing closer to explaining the mechanism behind this effect.

New research has started to unpick the mechanism that allows sleep to enhance memory consolidation.

It is known that sleep is vital to our normal daily functioning, and that a lack of proper nightly sleep can seriously affect our cognition and general well-being. For instance, Medical News Today recently covered a study that linked disturbed sleep with dementia, and another that investigated the correlation between attention deficit hyperactivity disorder and sleeplessness.

Yet many of the mechanisms underlying the impact of sleeping patterns on behavioral and cognitive disorders remain unknown.

Sleep falls into two categories: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. Scientists have also identified four different phases in which sleep occurs. Each of these is characterized by different brain activity types.

Stage one NREM. This refers to the transition from a state of wakefulness to a state of sleep, during which normal brain activity slows down. It is a stage of light sleep, from which one can easily be awoken.

Stage two NREM. This is also a stage of light sleep, yet it is slightly deeper than the one before. This stage is also transitional - this time from the light initial sleep to a state of deep sleep. It is characterized by short bursts of electrical activity, some of which are known as "sleep spindles."

Stage three NREM. This is a state of deep sleep during which brain activity slows even more. In this stage, the muscles fully relax, and heartbeat and breathing slow down. Immersion in this stage is associated with feeling refreshed the morning after.

REM. This is the stage of sleep during which dreaming happens. In this stage, brain activity patterns are mixed, with electrical brain waves sometimes peaking and sometimes slowing down.

Sleep spindles and 'adaptive brain changes'

Emerging research from the Humboldt and Charité Universities in Berlin, Germany, has looked at the mechanism that allows sleep spindles - a part of stage two NREM - to improve brain plasticity, or the brain's ability to adapt and reorganize by forming fresh neural connections.

"It is becoming increasingly clear," explains lead study author Dr. Julie Seibt, from the University of Surrey in Guildford, United Kingdom, "that sleep plays an important role in [the brain's] adaptive changes."

"Our study tells us that a large proportion of these changes may occur during very short and repetitive brain waves called spindles," she adds.

The researchers have recently published their findings in the journal Nature Communications.

Dendritic spikes tied to sleep spindles

Dr. Seibt and her colleagues performed their research first on mice who were exposed to 12-hour cycles of light alternated with dark and given free access to both food and drink, and then on freely behaving rats.

The researchers used electroencephalogram testing to record sleep spindles in stage two NREM. Also, since previous research indicated that dendritic calcium spikes were also associated with improved brain plasticity, the researchers set to measure those, too.

Dendrites are the branched receiving ends of neurons, which allow nerve cells to "pick up" electric signals, and the levels of calcium detectable at the intercellular level are indicators of neural activity.

Dr. Seibt and team found that dendritic calcium spikes increase in frequency during stage two of sleep, and that they are linked to sleep spindles, which have been associated with memory formation and learning.

This activity, however, was only observed in the cerebral cortex, which is the outer layer of the brain.

"Sleep spindles," explains Dr. Seibt, "have been associated with memory formation in humans for quite some time but nobody knew what they were actually doing in the brain."

"Now," she continues, "we know that during spindles, specific pathways are activated in dendrites, maybe allowing our memories to be reinforced during sleep."

The researcher hopes that these new insights into how sleep helps memory consolidation may eventually allow scientists to devise new ways of tackling memory disorders.

"In the near future, techniques that allow brain stimulation [...] could be used to stimulate dendrites with the same frequency range as spindles," she explains.

"This could lead to enhance[d] cognitive functions in patients with learning and memory disorders, such as dementia."

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